An Alzheimer's disease (AD) is a kind of degenerative brain diseases and is the cause of 60% of cases of dementia that leads to loss of cognitive ability due to a gradual degeneration of neurons. ADs are classified according to causes into familial ADs caused by genetic factors, and sporadic ADs that occur in a large number of patients even though exact causes of sporadic ADs are not known. When a brain tissue of a patient died from the AD is examined under a microscope, neuritic plaques (or senile plaques) and neurofibrillary tangles are observed as specific lesions. When the brain tissue is observed with naked eyes, signs of a global brain atrophy due to a loss of neurons are found.
The neuritic plaques are formed by accumulating proteins or dead cells outside cells, and include, as a main component, β-amyloid beta (Aβ) that denotes peptides of 42 or 43 amino acids. The neurofibrillary tangles are abnormal aggregates of hyperphosphorylated tau proteins in a cytoskeleton in a cell and look like balls of yarn.
An amyloid precursor protein (APP) gene, a tau gene and a presenilin 1 (PS1) gene known as typical genes responsible for the AD have been known to contribute to overexpression of β-amyloid and aggregation of a tau protein.
β-amyloid is generated by a proteolysis from an APP. The APP is a protein with a single transmembrane domain, is expressed as a few isotypes by alternative splicing, and is known to pass through two metabolic pathways in a cell. In one of the two metabolic pathways, p3 and sAPPα are generated by α-secretase and γ-secretase. In the other, β-amyloid and APPβ are generated by β-secretase and γ-secretase. In patients with the familiar AD, a mutation is found in the APP. Mutations, for example, a Swedish APP670/671 mutation, a Flemish APP672 mutation, a Florida APP716 mutation, a London APP717 mutation, and the like have been found, and an increase in formation of β-amyloid has been found in the mutations.
Also, the PS1 gene represents a mutation that causes the familiar AD. A PS1 is a protein with eight transmembrane domains, plays an important role in a generation process, and is known to act as γ-secretase or as a subunit of a γ-secretase complex. At least 45 mutations of the PS1 causing the familiar AD have been reported, and lead to an increase in an amount of β-amyloid to be formed.
The AD caused by generated β-amyloid leads to a process of a damage to neurons due to hyperphosphorylation of a tau protein. It has been known that a few phosphoenzymes act for the hyperphosphorylation of the tau protein. Due to formation of tangles of the tau protein in addition to the hyperphosphorylation of the tau protein, neurons are damaged. A mutation of the tau protein in which tangles are properly formed has been found.
Revealing of a tangle formation mechanism of a tau protein, and accumulation and aggregation of β-amyloid in addition to aging of human brain cells may be expected to play an important role in a treatment of the AD. Thus, there is a desire for a necessity to establish an animal model or a cell line in which an APP gene mutation, a PS1 gene mutation and a tau gene mutation are simultaneously expressed.
In various studies, attempts have been made to establish a transgenic mouse, to study a pathogenesis of the AD. A transgenic gene that is a main goal of the above attempts may include, for example, ApoE4, and an APP gene, a PS1 gene and a tau gene known as genes responsible for the familiar AD. A transgenic mouse that is being mainly used in a current study is a model to form neuritic plaques by increasing a concentration of β-amyloid in a brain using a mutation in an APP gene or a PS1 gene. However, since it is difficult to accurately know the pathogenesis of the AD based on only β-amyloid, attempts are being made to simultaneously insert mutant genes of a tau protein recently. The attempts are made to create a model more similar to a human AD by simultaneously expressing β-amyloid and the tau protein in a brain of a transgenic mouse.
When both a mutant APP gene and a mutant PS1 gene are present, β-amyloid may be generated even earlier. A phenomenon in which β-amyloid is accumulated in a brain of a first-generation transgenic mouse TG2576 begins after the transgenic mouse TG2576 is raised during at least 12 months, whereas an accumulation of β-amyloid is started within six months after a birth of a transgenic mouse 5XFAD or APP/PS1. A double transgenic mouse generated by mating a single APP transgenic mouse and a single PS1 transgenic mouse is being used in a variety of research, since 1996 when Duff succeeded in developing the double transgenic mouse. Actually, due to a synergistic effect of two genes in a double transgenic mouse, a spot is observed to be formed three months to nine months earlier. However, since most of double transgenic mice are obtained by mating an APP transgenic mouse and a PS1 transgenic mouse, expression of each gene is independently controlled from each promoter and used promoters are not expressed specifically to only neurons in numerous cases. Thus, it is difficult to conduct studies on the pathogenesis of the AD. In addition, metabolism of a mouse that is a rodent is greatly different from metabolism of a human, which may show a great difference in evaluation of effectiveness in development of medications for ADs.
To solve the above issues, AD mutant genes may be present in a single chromosome and may need to be completely linked and inherited to a next generation. Also, there is a desire for a necessity to use an animal that is very similar to a human and that may be freely transformed among non-rodents.